Your combo must be a fair way off being optimised (or is a mismatch of parts) if your 427 cube motor is lazy below 4000rpm.
A legit 650 to 700hp out of a single carb, aspirated, pump gas small block will need something much more serious than a hydraulic cam combo.
Buying new crank, rods, pistons and heads to get an extra 30ish cubes? Doesn't sound like good value. I'd sell it complete and put a big block in there. Solid roller P51 headed 557ish cube motor and you're in the high 700s without raising a sweat
